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Water splitting is the endergonic chemical reaction in which water is broken down into oxygen and hydrogen:
Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. A version of water splitting occurs in photosynthesis, but hydrogen is not released but rather used to drive the Calvin cycle. The reverse of water splitting is the basis of the hydrogen fuel cell. Water splitting using solar radiation has not been commercialized.
Production of hydrogen from water is energy intensive. Usually, the electricity consumed is more valuable than the hydrogen produced, so this method has not been widely used. In contrast with low-temperature electrolysis, high-temperature electrolysis (HTE) of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling fuel efficiency to about 50%. Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so the process is more efficient.
High-temperature electrolysis (also HTE or steam electrolysis) is a method for the production of hydrogen from water with oxygen as a by-product.
Photo-excitation of photosystem I initiates electron transfer to a series of electron acceptors, eventually reducing NADP+ to NADPH. The oxidized photosystem I captures electrons from photosystem II through a series of steps involving plastoquinone, , and plastocyanin. Oxidized photosystem II oxidizes the oxygen-evolving complex (OEC), which converts water into O2 and protons. Since the active site of the OEC contains manganese, much research has aimed at synthetic Mn compounds as catalysts for water oxidation.
In biological hydrogen production, the electrons produced by the photosystem are shunted not to a chemical synthesis apparatus but to , resulting in formation of H2. This biohydrogen is produced in a bioreactor.
Catalysis and proton-relay membranes are often the focus on development.
Other research includes thermolysis on defective carbon substrates, thus making hydrogen production possible at temperatures just under .
One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, a nuclear plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing grid energy storage schemes. As of 2005, there was sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants.
The hybrid thermoelectric copper–chlorine cycle is a cogeneration system using the waste heat from nuclear reactors, specifically the CANDU reactor supercritical water reactor.
Material constraints due to the required high temperatures are reduced by the design of a membrane reactor with simultaneous extraction of hydrogen and oxygen that exploits a defined thermal gradient and the fast diffusion of hydrogen. With concentrated sunlight as heat source and only water in the reaction chamber, the produced gases are very clean with the only possible contaminant being water. A "Solar Water Cracker" with a concentrator of about 100 m can produce almost one kilogram of hydrogen per sunshine hour.
The sulfur–iodine cycle (S–I cycle) is a series of thermochemistry processes used to produce hydrogen. The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The S–I process requires an efficient source of heat.
More than 352 thermochemical cycles have been described for water splitting by thermolysis. These cycles promise to produce hydrogen and oxygen from water and heat without using electricity. Since all the input energy for such processes is heat, they can be more efficient than high-temperature electrolysis. This is because the efficiency of electricity production is inherently limited. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient.
For all the thermochemical processes, the summary reaction is that of the decomposition of water:
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